RIBONUCLEIC
ACID (RNA)
RNA is a polymer of ribonucleotides
of Adenine, Uracil, Guanine and Cytosine joined together by 3’ – 5’ phosphodiester
bonds. Thymine is absent in RNA. RNA is found in the nucleolus, Nissl granules,
ribosomes, mitochondria and cytoplasm. The pentose sugar of the nucleotide is
D-ribose.
BIOLOGICAL ROLE OF RNA:
- RNA is the genetic material of some
viruses.
- RNA functions as the intermediate
(m-RNA) between the gene and the protein-synthesizing machinery.
- RNA functions as an adaptor (t-RNA)
between the codons in the mRNA and amino acids.
- RNA serves as a regulatory
molecule, which through sequence complementarity binds to, and interferes
with the translation of certain m-RNAs.
- Some RNAs are enzymes that catalyze
essential reactions in the cell (RNase P ribozyme, large rRNA,
self-splicing introns, etc).
STRUCTURE OF RNA:
a. Primary structure of
RNA:
·
The primary structure of RNA is defined
as the number and sequence of ribonucleotides in the chain.
·
Each linear strand is held together by the
ribonucleotides bound to each other by 3’-5’ phosphodiester bonds joining 3’
–OH of one nucleotide with the 5’ –OH of the next.
Figure 1: Primary structure of RNA
Figure 2: Secondary structure
of RNA (coils)
b. Secondary structure
of RNA:
·
The secondary structure of RNA involves
various coil formation of the polyribonucleotide chain.
·
These coils structures are stabilized by
hydrophobic interactions between the purine and pyrimidine bases.
·
There are intra-chain hydrogen bonds
between G-C and A-U. The hydrogen bonds are same as in DNA for G-C while N3
as well as C4 oxo group of uracil which pairs with adenine.
c. Tertiary Structure:
·
The tertiary structure of RNA involves
the folding of the molecule into three dimensional structures.
·
The cross linking also occurs at various
sites stabilized by hydrophobic and Hydrogen bonds producing a compactly coiled
globular structure.
Figure 3: G:U bonding in RNA structure
Figure
4: U:A:U triple base pairing in RNA
Figure
5:
Tertiary structure of RNA (Folding into 3-dimentional structure)
TYPES OF RNA:
There are mainly three
types of RNA. They are:
1.
Messenger RNA or m-RNA
2.
Transfer or soluble RNA or t-RNA and
3.
Ribosomal RNA or r-RNA
Figure 6:
Three different types of RNAs
The main functions of
each of these RNA are protein synthesis.
1. Messenger RNA or m-RNA:
This is the most heterogeneous
class of RNA with respect to its size and stability. The molecular weight
varies from 3 x 104 to 2 x 106. They consist of 103
to 104 ribonucleotides. It carries mainly adenine, guanine, cytosine
and uracil as the major bases and methylpurines and methylpyrimidines as minor
bases. The m-RNA molecules are formed with the help of DNA template strand
(3’-5’) during the process called transcription. The m-RNA carries a specific
sequence of nucleotides in triplets called “codons” responsible for the
synthesis of a specific protein molecule. The 3’-OH end of most m-RNA molecules
carries a polymer of adenylate ribonuclotides consisting of 20-250 residues in
length. This is called as Poly A tail, the function of which is not yet
fully understood but it seems to maintain the intracellular stability of the
specific m-RNA by preventing the attack of 3’-exonucleases. On the other hand,
the 5’-OH end of the m-RNA carries a cap structure consisting of 7
methylguanosine triphosphate. The cap is probably involved in recognition of
protein biosynthesis and it helps in stabilizing the m-RNA by preventing the
attack of 5’-exonucleases. The protein synthesis begins at 5’ end of the capped
structure of RNA.
Pre-mRNA
·
Pre-mRNA is conveted to m-RNA
·
Pre-mRNA has regions called as ‘introns’
transcripts the sequences not required (inactive) and ‘exons’
transcripts (active portion required for translation).
·
About 80 % of pre-mRNA is removed in
eukaryotes by RNA splicing mechanisms (no introns in prokaryotes).
Figure
7: Structure
of m-RNA
2. Transfer RNA or
t-RNA / s-RNA:
These are also called
soluble or s-RNA. They remain largely in cytoplasm. The t-RNAs are relatively
small, single stranded, globular molecules with molecular weight of 2 to 3 x 104.
These are at least 20 different types of t-RNA molecules.
a. Primary structure of
t-RNA
·
t-RNA consists of approximately 75
nucleotides. Their bases include adenine, guanine, cytosine, uracil,
pseudouridine or uracil 5-ribofuranoside and thymine are present in one loop.
b. Secondary structure
of t-RNA
·
Each single stranded t-RNA molecule
remains folded to form a clover leaf secondary structure. These folds of
the secondary structure are stabilized by H-bonds portions of the same strand. These
double bonded helical structures are called as stems.
All t-RNA molecules
consist of 4 main arms or loops.
1.
Acceptor arm:
This consists of unpaired sequence of cytosine-cytosine-adenine at the
3’ end also called as acceptor end. The 3’-OH terminal of adenine may bind with
the ยต–COOH
of a specific amino acid and carry the latter as an aminoacyl-t-RNA complex
to ribosomes for protein synthesis. The acceptor arm is borne by a base-paired
acceptor stem whose bases are hydrogen bonded with the last few bases at the 5’
end of t-RNA.
2.
Anticodon arm:
This is another unpaired and non bonded loop carrying specific sequences of
three bases constituting the anticodons. The bases of anticodon are hydrogen
bonded with three complementary bases of codon of m-RNA. The base pair stem
leading to anticodon loop is called anticodon stem.
3.
D arm:
The third is the D arm because it contains the base dihydrouridine.
4.
T y
C arm: Contains thymine, pseudouridine and cytosine.
5.
Variable arm or extra arm:
Extra arm is most variable arm and it forms the basis of classification.
T
y
C 7
bases
T
stem 5
bases pairs
D
arm 7-11
bases
D
stem 4
bases pairs
Anticodon
arm 7 bases
Anticodon
stem 5 bases pairs
Acceptor
arm 4 bases
Acceptor
stem 7 bases pairs
Variable
and Extra arm 4-21 bases
depending on the classes
Total
Approx 75
ribonucleotides
Figure 8: Amino acid sequences of t-RNA
Figure
9: Binding of m-RNA and amino acid with t-RNA (Aminoacyl-t-RNA complex)
3. Ribosomal RNA or
r-RNA
A ribosome is present
in the cytoplasm and is a nucleoprotein. It is on the ribosome that the m-RNA
and r-RNA interact during the process of protein biosynthesis. Ribosomes
contain the third type of RNA known as r-RNA. The r-RNA forms 80 % of the total
cellular RNA.
·
Ribosomes possess a sedimentation
coefficient of 80S in Eukaryotes and 70S in Prokaryotes. The 80S
ribosome contains subunits of 60S made up of 5S, 5.8S and 28S while 40 S
subunit 18S molecules. The 70S ribosome has subunits 50 S made up of 23S
and 5S while the smaller subunit 30 S has 16S molecules.
·
The main functions of r-RNA are in
assembly of ribosomal molecules and seem to play key roles in the binding of
m-RNA to ribosomes and its translation.
Figure
10: m-RNA,
t-RNA and r-RNA during the protein synthesis
Figure
11: Role
of RNAs in Translation process
DIFFERENTIATION OF DNA
AND RNA
DNA
|
RNA
|
Similarities:
|
1. Both have adenine, guanine and cytosine
|
2. The nucleotides are linked together by phosphodiester
bonds
|
3. The bonding is in 3’-5’ direction.
|
4. Main functions involve protein synthesis
|
|
Differences:
|
1. In addition to A, G, C the fourth base is T. Uracil is
absent.
|
1. In addition to A, G, C
the fourth base is U. Thymine is absent except in t-RNA.
|
2. Peotose sugar is deoxyribose.
|
2. Peotose sugar is ribose.
|
3. Present in nucleus, mitochondria, chloroplast and
cytoplasm
|
3. Present mainly in cytoplasm
|
4. Consists of 2 helical strands
|
4. Single stranded
|
5. There are
mainly A, B and Z forms of DNA
|
5. There are mainly m-RNA, t-RNA and r-RNA of RNA
|
6. Large molecules
|
6. Small molecules
except r-RNA
|
7. One strand 3’-5’ carries genetic information
|
7. m-RNA transcribed
from DNA carries genetic information
|
8. DNA can form RNA by the process of Transcription.
|
8. RNA cannot give rise
to DNA under normal conditions but it can give DNA under special process
using reverse transcriptase in some viruses.
|
9. Purine and pyrimidines contents are almost equal.
|
9. Purine and
pyrimidines contents are not equal.
|
Note: text is cited from Medical Biochemistry by Chaterjjee (Pictures are from different sources mentioned in pictures).
